Method of Drilling a Stable Borehole

ABSTRACT

A method of drilling a stable borehole in a formation containing a stress field having a direction of maximal stress, whereby the borehole is drilled with an elongated non-circular cross sectional contour along an axis of elongation and whereby a directional component of the axis of elongation is kept oriented substantially parallel to the direction of maximal stress.

BACKGROUND OF THE INVENTION

The present invention relates to a method of drilling a stable borehole in a formation in a formation containing a stress field having a direction of maximal stress.

Generally the earth formation surrounding the borehole is subjected to stresses including first, second and third principal stresses. One of these principal stresses is the largest of the three, thus the direction of maximal stress is along one of the three principal stress directions.

Often during drilling of a borehole, the formation rock in which a borehole is drilled may be threatened to collapse due to formation stress acting on the borehole walls. Collapse can be avoided by mounting casing or by choosing a sufficient mud weight of the mud that is circulated through the borehole. On the other hand, too high a mud weight results in a risk of loosing mud to the formation.

During production, reservoir rock can be loosely consolidated, so that it tends to disintegrate and flow into the wellbore under the influence of hydrocarbon fluid flowing through the pore spaces.

The possibility of such a collapse of the borehole or inflow of rock particles, the latter generally referred to as sand production, is a frequently occurring problem in the industry of hydrocarbon fluid production, as the produced sand particles tend to erode production equipment such as tubings and valves.

Conventional methods of sand control in reservoirs include the installation of supporting perforated liners or screens, which allow the hydrocarbon fluid to pass but exclude the sand particles. Also, gravel packs are installed between the liners or screens and the wellbore wall to control sand production. Although such liners, screens and gravel packs have often been successfully applied, there are potential drawbacks such as clogging of the perforations, screens or gravel packs leading to diminished fluid production.

U.S. Pat. No. 6,283,214 discloses a method of improving near wellbore stability and reduction of sand intrusion by making elliptically shaped perforations of a particular orientation into the well casing or formation.

U.S. patent application US 2003/0168216 discloses a method for reducing sand production by optimally orienting perforations.

It is an object of the invention to provide an improved method of drilling a borehole, which further enhances the wellbore stability and reduces sand intrusion and/or collapse of the borehole.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of drilling a borehole in a formation containing a stress field having a direction of maximal stress, whereby the borehole is drilled with an elongated non-circular cross sectional contour along an axis of elongation and whereby a directional component of the axis of elongation is kept oriented substantially parallel to the direction of maximal stress.

The presence of the borehole in the rock formation leads to stress concentrations in the rock formation region near the wellbore, when compared to the undisturbed rock formation. Such stress concentrations are believed to involve relatively high tangential stresses where tangential direction in the cross section of the borehole wall is approximately perpendicular to the direction of maximal stress in the formation. This can cause local compressive failure in some regions of the borehole wall and fracturing in others.

By orienting the direction of elongation of the borehole substantially parallel to the direction of maximal stress in the formation, it is achieved that the variation of tangential stress around the borehole contour is reduced. Particularly, the tangential stress, in those areas of the borehole where tangential direction in the cross section of the borehole wall is approximately parallel to the direction of maximal stress in the formation, reduces relative to the value of the maximal formation stress.

Thus the tendency of local rock formation failure and corresponding collapse, fracturing or sand production, is thereby reduced. Thereby a larger window becomes available for selecting the mud weight during drilling.

It is to be understood that the direction of elongation does not need to extend parallel to the longitudinal axis of the borehole, but can, for example, extend in the form of a helix along the wellbore wall depending on the variation of the formation stress field.

Suitably an elongate borehole can have a cross sectional contour with a circular section and an elongate section extending in a direction substantially parallel to the largest a selected one of said principal stresses.

In case the borehole extends substantially vertically into the formation, it is preferred that said axis of elongation extends in a direction substantially parallel to the largest horizontal principal stress.

In case the borehole extends substantially horizontally, it is preferred that said axis of elongation extends in a direction substantially parallel to the vertical principal stress.

It is possible to create a plurality of perforations in the wall of the borehole, said perforations forming a row extending in axial direction of the borehole. The perforations are closely spaced so as to form a pseudo-slot.

More preferably rock material is removed from each elongate section by creating a slot in the wall of the borehole, the slot extending in axial direction of the borehole. The slot can be wedge shaped in a cross-sectional plane of the wellbore, whereby the width of the slot decreases in radially outward direction.

However, preferably the elongate contour is made at the same time as progressing the borehole into the formation.

The borehole can be drilled for instance using a drill bit wherein two or more cutting sections are rotated instead of the entire drill bit.

Alternatively, the borehole can be drilled for instance using a down hole motor to rotate the drill bit or cutting sections thereof about an axis perpendicular to the longitudinal direction along which the borehole extends.

Alternatively, the borehole can be drilled for instance using a steerable drill bit that is brought to pendule reciprocably in a plane parallel to the longitudinal direction of the borehole.

Alternatively, jetting excavation or laser excavating may be employed to drill the borehole or to modify a circular borehole after it is drilled.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings in which:

FIGS. 1A to 1C schematically show cross sectional views of various elongate borehole cross sectional contours;

FIG. 3A schematically shows a wellbore in which an embodiment of the method of the invention is applied, at an initial stage of the method;

FIG. 3B shows the wellbore of FIG. 2A at a final stage of the method; and

FIG. 4 schematically shows a wellbore in which another embodiment of the method of the invention is applied.

DETAILED DESCRIPTION OF EMBODIMENTS SHOWN IN THE DRAWINGS

In the Figures, like reference signs relate to like components.

Referring to FIGS. 1A to C, there are shown examples of boreholes having various non-circular cross sectional contours 3 formed in a formation under stress.

FIG. 1A shows a borehole 40 having an elliptical or oval contour, FIG. 1B shows a borehole 50 having a lobed contour, and FIG. 1C corresponds to borehole 60 having a cross sectional contour with a circular section and an elongate section for instance in the form of slots. In each of these figures, one of the principal stress axes of the stress field in the formation extends perpendicular to the plane of the paper, and the other two are indicated by σ₁ and σ₂ whereby σ₂ corresponds to the direction of maximal stress. The shown borehole contours are elongate in the direction along an axis of elongation E. A directional component of the axis of elongation E is oriented substantially parallel to the direction of maximal stress σ₂.

In FIG. 2 is shown a circular cross-section of a wellbore section 30 which may be extending horizonally. The formation is subjected to in-situ stresses of which the principal stress σ₁ has the largest magnitude. This may be the vertical principal stress. The presence of the wellbore 30 in the formation 2 causes stress concentrations whereby the highest shear stresses occur near the wellbore wall, in point B about halfway the top and the bottom of the horizontal wellbore section 30.

When comparing the contours of FIGS. 1A to C to a circular contour as depicted in FIG. 2, the tangential stress concentrations in point B can be significantly reduced. In the case of a circular contour, the tangential stress in point B is approximately 3 times σ₁ due to an arching effect of the borehole wall whereas for an elliptical contour such as depicted in FIG. 1A the tangential stress in point B is approximately 1.2 to 2.0 times σ₁ where σ₁ indicates the maximum stress direction. Rock failure in point B is therefore less likely in the case of an elliptical contour than a circular contour.

For instance, in a horizontally extending borehole, the vertical stress corresponding to σ₁ could be 1 psi/ft, and the horizontal stress corresponding to σ₂ could be 0.9×σ₁. In the case of the circular contour of FIG. 2, such conditions could lead to a tangential stress concentration in point A of 1.7 psi/ft, and in point B of 2.1 psi/ft. The elliptical hole of FIG. 1A, taking a ratio of short axis over long axis of the ellipse of 0.5 could lead to a tangential stress concentration of 1.5 psi/ft in point A and of 1.1 in point B. Taking a ratio of short axis over long axis of 0.4, the tangential stress concentration in point A could be 1.07 and in point B 0.9 psi/ft. Hence, the stress concentration magnitudes are reduced and the stress concentration is more evenly distributed in the case of the elongate borehole contour.

Referring to FIG. 3A there is shown a borehole in the form of a wellbore 1 for the production of hydrocarbon fluid, the wellbore 1 extending into in an earth formation 2. An upper part of the bore wellbore 1 is provided with a casing 4 suspended from a wellhead 5 at the earth surface 6. The casing 4 is fixed in the wellbore by a layer of cement 7 located between the wellbore wall and the casing 4. The wellbore 1 has subsequently been drilled beyond the length of the casing 4 forming an open hole section of the wellbore 1. An injection string 8 for injecting cutting fluid extends from a drill rig 10 at surface, into the wellbore 1. The injection string 8 is at the lower end thereof provided with a fluid jet cutter 12 having a pair of jetting nozzles 14 oppositely arranged each other. The fluid jet cutter 12 is located near the lower end of the formation zone 3. Fluid jets 16 are ejected from the nozzles 14 against the wall of the wellbore 1 thereby creating slots 16 oppositely arranged in the wellbore wall.

In FIG. 3B is shown the wellbore 1 after the injection string 8 has been raised to a position whereby the fluid jet cutter 12 is located near the upper end of the formation zone 3. The slots 16 extend in axial direction 17 of the wellbore 1 and along substantially the whole length of the open hole section of the wellbore 1.

It is remarked that the open hole section of the wellbore may reach into a formation zone containing a hydrocarbon fluid to be produced.

During normal use the wellbore 1 is drilled to a certain depth, the casing 4 is installed, and cement is pumped between the casing 4 and the wellbore wall to form the layer of cement 7. Subsequently the wellbore 1 is further drilled to form a so-called open hole section. The injection string 8 is lowered into the wellbore 1 such that the jet cutter 12 is located near the bottom of the wellbore 1 (FIG. 3A). Cutting fluid (e.g. water or an abrasive particle containing mixture) is then pumped through the string 8, so as to induce the fluid jet cutter to jet two opposite jet streams against the wellbore wall. As a result slots 16 are created in the wellbore wall. Simultaneously with pumping cutting fluid through the string 8, the string is gradually raised in the wellbore 1 until the jet cutter 12 is located near the top of the open hole section near the casing 4. Thus the slots 16 are formed along substantially the whole length of the open hole section of the wellbore 1 below the casing 4.

In the embodiment shown in FIG. 3, the jet cutter 12 is kept oriented in the wellbore 1 such that the nozzles 14 are positioned in along the direction of the maximal stress in the formation.

Instead of creating slots or rows of perforations, in the open-hole section of a wellbore, such slots or rows of perforations suitably can be formed in the rock formation behind a perforated liner or casing.

Instead of creating the slots using the jet cutter described hereinbefore, the slots can be created by a mechanical device such as a chain saw, or by an explosive charge.

A preferred embodiment of the method is illustrated with reference to FIG. 4. In this case, a specially adapted drill bit 22 is employed on a lower end of a drill string 28. The drill bit is provided with two cutting sections 22A and 22B, which can each be rotated instead of the entire drill bit 22. In order to allow drill string rotation of the drill string 28, a clutch unit 25 can be employed that uncouples drill string 28 rotation from rotation of the drill bit 22. A downhole motor unit 24 can be employed to drive the cutting sections 22A and 22B.

By drilling the hole with the drill bit 22 as depicted, it is expected that a lobed borehole cross-section can be drilled such as is shown in FIG. 1B, whereby reference numeral 26 corresponds to the elongation in the form of lobes.

An advantage of this embodiment over the embodiment of FIG. 3 is that the elongate contour is made at the same time as progressing the borehole into the formation.

This advantage would also be achieved by using a steerable drill bit that is brought to pendule reciprocably in a plane parallel to the longitudinal direction 17 of the borehole. Such a method would result in an oval borehole contour such as is depicted in FIG. 1A.

The method as illustrated in FIG. 3 can be used to form any of the contours of FIG. 1, including the slotted contour of FIG. 1C. 

1. A method of drilling a borehole in a formation containing a stress field having a direction of maximal stress, wherein a portion of the borehole is formed with an elongated non-circular cross section having an axis of elongation and wherein the axis of elongation is substantially parallel to the direction of maximal stress.
 2. The method of claim 1 wherein each said elongate portion has a longitudinal axis extending in axial direction of the borehole.
 3. (canceled)
 4. (canceled)
 5. The method of claim 3 wherein the wellbore extends substantially vertically, and wherein said elongate portion extends radially in a direction substantially parallel to the largest horizontal principal stress.
 6. The method of claim 3 wherein the wellbore extends substantially horizontally, and wherein said elongate portion extends radially in a direction substantially parallel to the vertical principal stress.
 7. The method of claim 1 wherein during drilling the elongate cross section is made at the same time as progressing the borehole into the formation.
 8. A borehole in a formation containing a stress field having a direction of maximal stress, wherein whereby the borehole has an elongated non-circular cross sectional contour having an axis of elongation, and wherein the axis of elongation is oriented substantially parallel to the direction of maximal stress. 